In an optical communication system, an optical signal transmitted from an optical transmitter is propagated in an optical fiber as a transmission path and is transmitted to an optical receiver. At this time, an optical amplifier is used to compensate a loss generated in the transmission path. A postamplifier allocated on an output side of the optical transmitter, a preamplifier allocated in front of the optical transmitter, or an inline amplifier that is used for multiple relay of an optical signal is given as an example of allocation of the optical amplifier. For example, an Erbium-Doped Fiber Amplifier (EDFA) using an erbium-doped fiber as an optical amplification medium is widely used in an optical communication system of 1550 nm band.
The optical amplifier outputs an amplified signal light and an amplified spontaneous emission (ASE). Since the ASE is randomly generated to interfere with the signal light, the ASE may be a noise component in the optical communication system. Since the property of the optical receiver is limited according to the noise component, it is important to measure the signal component and the noise component in the optical communication system. Usually, the characteristic of the optical receiver is expressed in resistance of an optical signal to noise ratio (OSNR).
In a broadband access network, a larger capacity may be achieved by applying a Wavelength Division Multiplexing (WDM) technique according to increase of Internet traffic. A transmission rate per one wavelength in a WDM optical communication system is shifted from 2.5 Gbps or 10 Gbps to 40 Gbps or 100 Gbps. In the WDM optical communication system in which the transmission rate is equal to or more than 40 Gbps, the interval between optical pulses of 1 bit width is narrower, and the spectrum width (frequency band) of the signal light is wider compared to a case of the transmission rate of 10 Gbps. Due to this, deterioration of the transmission character is increased by influence of the noise of the optical amplifier, and wavelength dispersion and polarization dispersion of the transmission path optical fiber. As a result, a phase modulating method such as, for example, Differential Phase Shift Keying (DPSK) and Differential Quadrature Phase Shift Keying (DQPSK) that have excellent noise resistance and dispersion resistance is employed as a phase modulating method that is different from a conventional On-Off Keying (OOK) method.
FIG. 1 is a diagram illustrating an example of a related technique of an output spectrum of the EDFA used in the WDM optical communication system. As illustrated in FIG. 1, in the EDFA, a plurality of signal lights allocated in 1550 nm band is amplified, and a wideband ASE of approximately 40 nm is generated. In the WDM optical communication system in which a plurality of EDFAs is allocated on a transmission path to perform multiple relay transmission of WDM light, ASEs are accumulated every time the WDM light goes through the EDFA. The WDM light on which the ASEs are accumulated is branched into lights of each wavelength by the branch unit allocated in the reception end and is then input into the optical receiver corresponding to each wavelength to be subjected to the receiving processing.
FIG. 2 is a diagram illustrating a related art example of an optical spectrum of one wavelength that is output from the branch unit. In this manner, the optical receiver corresponding to each wavelength receives each light that includes a spectrum in which the signal light component and the ASE component are superimposed. The OSNR [dB] of the reception light is expressed by the formula (1) if the power of the signal light component is Psig [mW] and the power of the ASE component of 0.1 nm bandwidth in the signal optical wavelength.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      OSNR          ⁡                      [            dB            ]                          =                  10          ×                      log            ⁡                          (                                                P                  sig                                                                      P                    ase                                    ,                                      0.1                    ⁢                                                                                  ⁢                    nm                                                              )                                                          (        1        )            
Therefore, to obtain the OSNR, the power of the signal light component and the power of the ASE component included in the reception light are desired to be measured. If the spectrum of the reception light is measured by using an optical spectrum analyzer or the like, the powers are measured where the signal optical component overlaps with the ASE component on the same wavelength (see FIG. 2). As a result, the OSNR may not be obtained. To separate the signal optical component from the ASE component, for example, various techniques such as a pulse technique, an ASE interpolation technique, a probe technique, a polarization extinction ratio technique, and a time-domain technique, which are described in Yoshinori Namihira, “DWDM Optical Fiber Measurement Technologies,” The Optronics Co., Ltd., pp. 95 to 100, March 2001, have been applied.
Specifically, the pulse technique is used to measure a spectrum of the output light by turning on and off the input light to the optical amplifier by using an optical switch or the like and measuring an ASE level when the input light is turned off (see, for example, Japanese Patent No. 3467296). In the ASE interpolation technique, the spectrum of the output light of the optical amplifier is measured, the ASE part is taken out from the spectrum, and the level of the ASE component that overlaps with the signal optical component on the same wavelength is estimated (see, for example, Japanese Patent No. 3311102).